The goal of this proposal is to elucidate the mechanisms by which loss of Runx1 in hematopoietic stem and progenitor cells alters their function, and contributes to the development of myelodysplastic syndrome (MDS) and acute myelogenous leukemia (AML). Inactivating mutations in RUNX1 are found in a high percentage of patients with AML of the least differentiated M0 subtype, and in MDS. MDS is a collection of blood disorders characterized by the inefficient production of one or more blood cell lineages, for which there is currently no cure aside from bone marrow transplantation. Loss of Runx1 in mice appears to recapitulate early stages of MDS, and is therefore a model for studying how impaired Runx1 function contributes to the development of this disease. We showed that loss of Runx1 causes hematopoietic stem cells and multipotent progenitors to proliferate more frequently, and that the expression of approximately 2000 genes and 40 microRNAs is altered in these populations. Here we propose to characterize the defects in HSCs and progenitors caused by Runx1 deficiency. We will examine whether genes whose expression is dysregulated in Runx1 deficient HSCs contribute to these defects. We will determine if treatment of mice with an agonist of a cytokine receptor that is under-expressed improves engraftment. Finally we will assess the contribution of several microRNAs to the engraftment defects associated with Runx1 deficiency, and their ability to cause MDS.